Chapter 3: Data Modeling Using the Entity-Relationship (ER) Data Model

1. Database Design Process2. Example Database Application (COMPANY)3. ER Model Concepts 3.1 Entities and Attributes 3.2 Entity Types, Value Sets, and Key

Attributes 3.3 Relationships and Relationship Types 3.4 Structural Constraints and Roles 3.5 Weak Entity Types4. ER Diagrams Notation5. Relationships of Higher Degree6. Extended Entity Relationship (EER) Model

2. Example: Company Database• Requirements for the Company Database:

• The company is organized into DEPARTMENTs. Each department has a name, number, and an employee who manages the department. We keep track of the start date of the department manager. A department may have several locations.

• Each department controls a number of PROJECTs. Each project has a name, number, and is located at a single location.

• We store each EMPLOYEE's social security number, address, salary, sex, and birth date. Each employee works for one department but may work on several projects. We keep track of the number of hours per week that an employee currently works on each project. We also keep track of the direct supervisor of each employee.

• Each employee may have a number of DEPENDENTs. For each dependent, we keep their name, sex, birth date, and relationship to the employee.

3. ER Model Concepts3.1 Entities and Attributes

• Entities are specific objects or things in the mini-world that are represented in the database; for example the EMPLOYEE John Smith, the Research DEPARTMENT, the Product X PROJECT.

• Attributes are properties used to describe an entity; for example an EMPLOYEE entity may have a Name, SSN, Address, Sex, BirthDate.

• A specific entity will have a value for each of its attributes; for example a specific employee entity may have Name='John Smith', SSN='123456789', Address='731 Fondren, Houston, TX', Sex='M', BirthDate='09-JAN-55'.

Types of Attributes• Simple: Each entity has a single atomic value for the

attribute; for example SSN or Sex.

• Composite: The attribute may be composed of several components; for example Address(Apt#, House#, Street, City, State, ZipCode, Country) or Name(FirstName, MiddleName, LastName). Composition may form a hierarchy where some components are themselves composite.

• Multi-valued: An entity may have multiple values for that attribute; for example Color of a CAR or PreviousDegrees of a STUDENT. Denoted as {Color} or {PreviousDegrees}.

• In general, composite and multi-valued attributes may be nested arbitrarily to any number of levels although this is rare. For example, PreviousDegrees of a STUDENT is a composite multi-valued attribute denoted by {PreviousDegrees(College, Year, Degree, Field)}.

3.2 Entity Types and Key Attributes

• Entities with the same basic attributes are grouped or typed into an entity type. For example, the EMPLOYEE entity type or the PROJECT entity type.

• An attribute of an entity type for which each entity must have a unique value is called a key attribute of the entity type. For example SSN of EMPLOYEE.

• A key attribute may be composite. For example, VehicleRegistrationNumber is a key of the CAR entity type with components (Number, State).

• An entity type may have more than one key. For example, the CAR entity type may have two keys: VehicleIdentificationNumber and VehicleRegistrationNumber(Number, State).

Name Age Salary Name Headquarter President

EMPLOYEE COMPANY

3.3 Relationships and Relationship Types

• A relationship relates two or more distinct entities with a specific meaning; for example:

EMPLOYEE John Smith works on the Product X PROJECT EMPLOYEE Franklin Wong manages the Research

DEPARTMENT.

• Relationships of the same type are grouped or typed into a relationship type. For example:

the WORKS_ON relationship type in which EMPLOYEEs and PROJECTs participate

the MANAGES relationship type in which EMPLOYEEs and DEPARTMENTs participate.

• The degree of a relationship type is the number of participating entity types. Both MANAGES and WORKS_ON are binary relationships.

• More than one relationship type can exist with the same participating entity types; for example:

MANAGES and WORKS_FOR are distinct relationships between EMPLOYEE and DEPARTMENT participate.

Binary Relationship

Ternary Relationship

Recursive Relationship

3.4 Structural Constraints and Rolesof Relationships

• A relationship can relate two entities of the same entity type ; for example, a SUPERVISION relationship type relates one EMPLOYEE (in the role of supervisee ) to another EMPLOYEE (in the role of supervisor ). This is called a recursive relationship type.

• A relationship type can have attributes; for example, HoursPerWeek of WORKS_ON; its value for each relationship instance describes the number of hours per week that an EMPLOYEE works on a PROJECT.

• Structural constraints on relationships:• Cardinality ratio (of a binary relationship): 1:1, 1:N, N:1, or M:N.

• Participation constraint – Tota: All entities are covered– Partial: Some entities are not covered

Alternative (min, max) notation for relationship structural constraints:

• Specified on each participation of an entity type E in a relationship type R.• Specifies that each entity e in E participates in at least min and at most

max relationship instances in R.• Default (no constraint): min=0, max=n.• Must have min<max, min>0, max>1.• Derived from the mini-world constraints.• Examples:• (a) A department has exactly one manager and an employee can manage

at most one department.– Specify (1,1) for participation of DEPARTMENT in MANAGES– Specify (0,1) for participation of EMPLOYEE in MANAGES

• (b) An employee can work for exactly one department but a department can have any number of employees.– Specify (1,1) for participation of DEPERTMENT in WORKS_FOR– Specify (0,n) for participation of EMPLOYEE in WORKS_FOR

3.4 Weak Entity Types• An entity type that does not have a key attribute of its

own• A weak entity type must participate in an identifying

relationship type with an owner or identifying entity type

• Entities are identified by the combination of :-A partial key of the weak entity type and-The key of a particular entity they are related

to in the identifying entity typeExample:

• Suppose that a DEPENDENT entity is identified by the dependent's first name and birthdate, and the specific EMPLOYEE that the dependent is related to.DEPENDENT is a weak entity type with EMPLOYEE as its identifying entity type (strong entity type) via the identifying relationship type DEPENDENT_OF.

5. Relationships of Higher Degree

• Relationship types of degree 2 are called binary

• Relationship types of degree 3 are called ternary

• Relationship types of degree n are called n-ary

• In general, an n-ary relationship is not equivalent to n binary relationships

HW #2

Do exercise 3.23Due Jan 29.